The invention relates to linear actuators. A majority of linear actuators include motor/gearbox/spindle+nut arrangements. Normally when an external force is applied on an inactive (stopped) actuator—the mechanic self-lock created by gearbox and spindle will make actuator hold its position.
However, when external load is dynamic and also shifts direction (push to pull to push), the self-lock is not 100% reliable, especially if these dynamic forces are present for longer periods of time.
In order to solve this, many companies offer brakes as bolt-on options on actuators, but this solution has drawbacks. For example cost, increased size and increased power consumption. Also it is normal that the brake is based on an electromagnet which also requires special interfaces to the control system.
From WO2006048023 there is known magnetic brake device that may be used to safely position a table top, e.g. to eliminate a table top from being lowered by its own weight. The magnetic brake device comprises a rotor part that is produced in one piece and that has cruciform cross-section made of a magnetizable material, e.g. iron. Such a solution is deficient in applications where frequent vibrations and/or abrupt movements frequently occur in the vertical. Seats used in vehicles is a typical application of that kind, e.g. tractors, wheelchairs, etc. where relatively large gravitational forces applied to the seat and wherein the ground upon which the vehicle travels will cause frequently occurring vertical movements and thereby also forces. Such forces may be of a relatively large magnitude implying that the magnetic brake must be able to withstand relatively large torque. At the same time it is desired to keep the mass of the magnetic brake within acceptable limits as well as the size thereof in order to not compromise regarding output or compactness.
Further, from U.S. Pat. No. 5,751,127A a magnetic brake is disclosed of a multi-pole magnet assembly kind, suggested for a specific kind of applications having a pivotal device to lock, e.g. a retractable landing light assembly of an aircraft, to eliminate the risk that the lowered landing lights may be forced back into the plane from the high torque caused by the wind outside the plane.